Duct static pressure control

ABSTRACT

A method of controlling a variable-air-volume heating, ventilating, and air-conditioning system, includes causing the static pressure in a duct to be maintained at a desired static pressure by controlling the speed of a fan in proportion to the square root of the ratio of a desired duct static pressure setpoint to a measured duct static pressure. A HVAC system employing the method is further included.

RELATED APPLICATION

The present application claims the benefit of U.S. Provisional Application No. 60/873,306 filed Dec. 6, 2006, which is incorporated herein in its entirety by reference.

FIELD OF THE INVENTION

The following invention relates to controls for variable-air-volume (VAV) heating, ventilating, and air-conditioning (HVAC) systems, specifically to control the speed of a supply fan in VAV HVAC systems.

BACKGROUND OF THE INVENTION

Modern buildings typically have complex heating, ventilating, and air-conditioning (HVAC) systems to control indoor temperature, pressure, ventilation rate, and other variables in a way that makes efficient use of energy. One way to conserve energy in these systems is to use a so-called variable-air-volume (VAV) design. Key components of a variable-air-volume system are a supply fan and terminal units. The supply fan is a prime mover that causes air to move. A terminal unit contains a throttling device that regulates an amount of air supplied to a space in a building that it controls in order to regulate temperature and ventilation in that space. The supply fan is typically in communication with a plurality of terminal units.

In a variable-air-volume system the flow rate of conditioned air supplied to a building is adjusted so that no more air than necessary is used. Variable flow is achieved by means of the use of controls on or near the supply fan and by use of controls on the terminal units. The supply fan controls vary the speed of the supply fan to provide efficient airflow modulation. The controls on the terminal units determine how much air flows through each terminal.

The most common control strategy for the supply fan of variable-air-volume systems is to regulate a static pressure in a supply duct at a point downstream of the supply fan before the terminal unit(s), although static pressure measurement may be made downstream of one or more of the terminal ducts. A rule of thumb is to locate the pressure sensor two-thirds of the distance from the supply fan to the end of the supply duct. The airflow is varied so that the static pressure is maintained at a setpoint that may be constant or reset based on airflow.

Control strategies based on a constant static pressure in the supply duct have been proposed in U.S. Pat. No. 4,437,608 to Smith (1984) and U.S. Pat. No. 6,227,961 to Moore et al. (2001).

Control strategies that reset the static pressure based on the position of the terminal unit that is most open have been proposed in U.S. Pat. No. 5,863,246 to Bujak (1999). An objective of these strategies is to keep the terminal unit damper nearly open or completely open. Doing this reduces the throttling losses at part-load conditions.

A control strategy that adjusts the static pressure setpoint based on measured airflow was proposed as the preferred embodiment in U.S. Pat. No. 6,719,625.

All of these strategies to determine a setpoint require a method of adjusting the volume of air provided by the supply fan to maintain the measured duct static pressure at the desired setpoint.

Traditionally a controller uses the Proportional-Integral-Derivative (PID) control algorithm. This algorithm, which takes many forms, is based on the difference between the duct static pressure setpoint and actual duct static pressure usually referred to as the Error.

A PID algorithm changes the airflow as needed to minimize the Error. These periodic changes are based on this Error and how the Error is changing. The supply fan may be driven by a Variable Frequency Drive. When the airflow modulating device is a Variable Frequency Drive, the calculated speed change is the same regardless of the fan speed for any set of conditions. Fan laws dictate that the duct static pressure varies with the square of the fan speed. A speed change at low speeds results in a bigger change in the duct static pressure than the same speed change at a higher speed. As a result, a PID loop tuned to provide a good response at low speeds will be more sluggish at high speeds and a PID loop tuned to provide a good response at high speed may hunt at low speeds.

PID parameters must be adjusted, or tuned, for the algorithm to provide acceptable operation. Such tuning can be difficult with any implementation of a PID loop. The problem of tuning is made more difficult because of the multiple forms that PID algorithms may take. For example, a loop may respond more quickly when the Integral parameter is increased in one implementation and it may respond more slowly when the Integral parameter is increased in another implementation.

Accordingly, a need exists for an algorithm that changes the speed of a variable frequency drive more at higher speeds for a given set of circumstances so that the pressure change is nearly the same at all speeds for a given set of circumstances. In addition, a need exists for an algorithm that does not require tuning of control parameters of a variable frequency drive to maintain a desired duct static pressure.

SUMMARY OF THE INVENTION

In accordance with the present invention, a control algorithm is used to control the speed of the supply fan of a variable-air-volume heating, ventilating, and air-conditioning system comprised of the supply fan, a variable frequency drive, a static pressure sensor, and a controller coupled to the static pressure sensor and variable frequency drive. The controller causes the static pressure downstream of the fan to be maintained at a desired static pressure by causing the speed of the variable frequency drive to vary when necessary in proportion to the square root of the ratio of the desired duct static pressure setpoint to the measured duct static pressure.

The algorithm of the present invention changes the speed of a variable frequency drive more at higher speeds for a given set of circumstances so that the pressure change is nearly the same at all speeds for a given set of circumstances. In addition, the algorithm of the present invention does not require tuning of control parameters of any certain variable frequency drive to maintain a desired duct static pressure.

Accordingly, a primary object of the current invention is to provide more accurate and quicker responding control of the duct pressure at a desired duct static pressure in a variable-air-volume system in which a variable frequency drive is used to set the speed of the supply fan.

Another object of the current invention is to reduce or eliminate the need to tune parameters to control a variable frequency drive to maintain the duct pressure at a desired duct static pressure in a variable-air-volume system.

Then present invention is a method of controlling a variable-air-volume heating, ventilating, and air-conditioning system and includes causing the static pressure in a duct to be maintained at a desired static pressure by controlling the speed of a fan in proportion to the square root of the ratio of a desired duct static pressure setpoint to a measured duct static pressure. Then present invention is further a HVAC system employing the above method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a portion of a variable-air-volume (VAV) heating, ventilating, and air-conditioning (HVAC) system.

FIG. 2 is a block diagram of the control method for changing the speed of the supply fan to maintain the measured duct static pressure close to the desired duct static pressure setpoint.

DETAILED DESCRIPTION OF THE DRAWINGS

The embodiment of the method of adjusting the speed of a variable frequency drive to maintain a desired duct static pressure is shown in FIG. 1 and FIG. 2.

Physical components of the system in which this method is used include the supply fan 1, a supply duct 2, two or more terminal ducts 3, two or more terminal units 4, a static pressure sensor 5, a supply fan controller 6, and a variable frequency drive 7. The system also contains other components such as heat exchangers, cooling coils, and filters not shown in FIG. 1 that are used for other functions such as heating, cooling, and cleaning air. The supply fan 1 could be either a centrifugal fan or an axial fan. The supply fan duct 2 is an elongate sheet metal structure typically with rectangular cross-section used to transport air. Each terminal duct 3 is an elongate structure made of sheet metal or other material that transports air to a terminal unit 4. The static pressure sensor 5 is located downstream of the supply fan 1. The static pressure sensor 5 indicates the static pressure in the supply duct 2. The supply fan controller 6 is an electronic device with microprocessor and memory. The controller 6 may be integrated with the variable frequency drive 7. A unique feature of the present invention is the algorithm used by the supply fan speed calculator 9 to determine the speed of the variable frequency drive 7 by comparing the output of the static pressure sensor 5 to the desired static pressure setpoint 8.

A signal from the static pressure sensor 5 and a static pressure setpoint 8 are inputs to the supply fan speed calculator 9. The setpoint 8 may be a fixed value manually entered into the controller 6 or it may be automatically varied as airflow requirements change by means either internal or external to the supply fan controller 6. The output of the supply fan controller 6 is the input to the variable frequency drive 7.

Operation

In operation, the calculated speed at which the supply fan 1 should operate for any given conditions is determined by using the known non-linear fan law relationship between pressure and fan speed to provide improved control.

Fan laws that relate performance variables are described in the ASHRAE Systems and Equipment Handbook. For a particular fan at a constant density, fan law #2 b relates speed, N, to pressure, P, in the following manner.

N ₂ =N ₁×(P ₂ /P ₁)^(1/2)

Referring to Table 1, to determine the speed, N₂ required to achieve the desired duct static pressure, P₂, the current Speed, N₁, provided through a variable frequency drive 7 is multiplied by an estimate of the square root of the ratio of the duct static pressure setpoint, P₂, divided by the current duct static pressure, P₁, as indicated in Table 1. The speed, N, is changed to the new calculated speed, N₂ whenever the duct static pressure, P₁ differs from the duct static pressure setpoint, P₂, by more than a fixed amount, and the variable frequency drive speed has not been changed for longer than the current sample time.

The fan laws apply to steady state situations so the current sample time is adjusted when each speed change calculation is made so that the duct static pressure is close to its steady state value when the next calculation is made. One way to accomplish this is by first multiplying the change in speed expressed as a percent of the maximum variable frequency drive speed by the acceleration/deceleration time for the variable frequency drive and then adding an additional time period to allow the unit time to stabilize after the speed change.

The absolute value of the speed change is limited to a maximum value so that timely speed changes can be made when the duct static pressure is relatively far from the desired setpoint that would result in large calculated speed changes and sample times.

The speed is also set to this maximum value to provide a positive fan speed when the current fan speed is zero.

TABLE 1 DUCT STATIC PRESSURE CONTROL METHOD Definitions DSP = Duct Static Pressure Speed = Fan Speed signal sent to VFD Edited Values DSP_Spt = Duct Static Pressure Setpoint Acc/Dec Time = Time required for VFD to change speed of fan from 0 to 100% or 100% to 0. DSP_Deadband = Static Pressure range above and below setpoint when no changes are made. MaxVFDStep = Maximum allowed change in Speed Algorithm Speed Change is calculated continuously SP_Ratio = DSP_Spt/DSP Sqrt = Square Root of SP_Ratio Speed Change = (Sqrt − 1) × Speed If Speed Change > MaxVFDStep then Speed Change = MaxVFDStep If Speed Change < −MaxVFDStep then Speed Change = −MaxVFDStep If Speed = 0 then Speed Change = MaxVFDStep A new Wait Time is calculated whenever the speed is actually changed (Absolute Value of Speed Change) × Acc/Dec Time = Speed Change Time Wait Time = Speed Change Time + Stabilize Time Timer is reset to zero whenever the speed is actually changed Speed is actually changed to Speed + Speed Change when: Timer > Wait Time AND (Absolute value of DSP_Spt − DSP) > DSP_Deadband/2

The method of Table 1 requires no changes to existing VFD's and no additional wiring to what is standard in VAV units. An advantage to this approach is that existing components are usable and that existing VAV units can be updated with the algorithm of Table 1.

Table 2 describes an alternate way of deciding when to change the speed that could be implemented if the actual speed at which the fan is being driven were known. It should be noted that the speed change is calculated as noted above in Table 1. This requires the use of another analog input and wiring between the VFD and the controller. See line 10 of FIG. 1. This algorithm may be implemented within a VFD by a VFD manufacturer.

TABLE 2 ALTERNATIVE DUCT STATIC PRESSURE CONTROL METHOD Alternative method of determining when Speed is actually changed VFD Feedback = signal from VFD to Controller indicating actual speed at which Fan is being driven. This value will increase or decrease to the Fan Speed signal sent to VFD after Speed is changed. Speed is actually changed to Speed + Speed Change when: VFD Feedback = Speed for longer than Stabilize Time AND (Absolute value of DSP_Spt − DSP) > DSP_Deadband/2

While the duct static pressure control of the present invention has been shown and described in detail, the invention is not to be considered as limited to the exact forms disclosed, and changes in detail and construction may be made therein within the scope of the invention without departing from the spirit thereof. 

1. A method of adjusting the speed of a supply fan to maintain the static pressure in a supply duct at a desired pressure setpoint in a variable-air-volume heating, ventilating, and air-conditioning system comprised of the supply fan, a variable frequency drive, a static pressure sensor, and a controller coupled to the static pressure sensor and the variable frequency drive, the method comprising: varying the speed of the supply fan in proportion to the square root of the ratio of the desired duct static pressure setpoint to the current measured duct static pressure.
 2. The method of claim 1, including changing the fan speed to the new calculated speed whenever the actual duct static pressure differs from the duct static pressure setpoint by more than a known amount and the variable frequency drive speed has not been changed for longer than a known current sample time.
 3. The method of claim 1, including changing the fan speed to the new calculated speed whenever the fan speed is stabilized.
 4. The method of claim 3, including determining fan speed stabilization by multiplying the change in speed expressed as a percent of the maximum variable frequency drive speed by the acceleration/deceleration time for the variable frequency drive and then adding an additional known sample time period.
 5. The method of claim 4, including limiting the absolute value of the speed change of the fan to a maximum value.
 6. The method of claim 5, including limiting the absolute value of the speed change of the fan to a maximum value is limited to a known maximum value so that timely speed changes can be made when the duct static pressure is relatively far from the desired setpoint.
 7. The method of claim 6, including setting the speed to the known maximum value for providing a positive fan speed when the current fan speed is zero.
 8. The method of claim 1, including changing the speed of a variable frequency drive more at higher fan speeds than at lower fan speeds so that the pressure change is nearly the same at all speeds for a given set of circumstances.
 9. The method of claim 1, including not tuning control parameters of a certain variable frequency drive to maintain a desired duct static pressure.
 10. A method of controlling a variable-air-volume heating, ventilating, and air-conditioning system, comprising: causing the static pressure in a duct to be maintained at a desired static pressure by controlling the speed of a fan in proportion to the square root of the ratio of a desired duct static pressure setpoint to a measured duct static pressure.
 11. The method of claim 10, including changing the fan speed to the new calculated speed whenever the actual duct static pressure differs from the duct static pressure setpoint by more than a known amount and the variable frequency drive speed has not been changed for longer than a known current sample time.
 12. The method of claim 10, including changing the fan speed to the new calculated speed whenever the fan speed is stabilized.
 13. The method of claim 12, including determining fan speed stabilization by multiplying the change in speed expressed as a percent of the maximum variable frequency drive speed by the acceleration/deceleration time for the variable frequency drive and then adding an additional known sample time period.
 14. The method of claim 10, including limiting the absolute value of the speed change of the fan to a maximum value.
 15. The method of claim 14, including limiting the absolute value of the speed change of the fan to a maximum value is limited to a known maximum value so that timely speed changes can be made when the duct static pressure is relatively far from the desired setpoint.
 16. The method of claim 15, including setting the speed to the known maximum value for providing a positive fan speed when the current fan speed is zero.
 17. A HVAC system, comprising: a method of control, the method causing the static pressure in a duct to be maintained at a desired static pressure by controlling the speed of a fan in proportion to the square root of the ratio of a desired duct static pressure setpoint to a measured duct static pressure.
 18. The HVAC system of claim 17, the method including changing the fan speed to the new calculated speed whenever the actual duct static pressure differs from the duct static pressure setpoint by more than a known amount and the variable frequency drive speed has not been changed for longer than a known current sample time.
 19. The HVAC system of claim 17, the method including changing the speed of a variable frequency drive more at higher fan speeds than at lower fan speeds so that the pressure change is nearly the same at all speeds for a given set of circumstances.
 20. The HVAC system of claim 19, the method including not tuning control parameters of a certain variable frequency drive to maintain a desired duct static pressure. 